1. Field of the Invention
This invention relates to a method for preparing an antibacterial/antifungal inorganic matter-composited wood and more particularly, to a method for preparing an antibacterial/antifungal inorganic matter-composited wood having an antibacterial/antifungal component incorporated therein in such a manner that the antibacterial/antifungal component is prevented from being leached out in water and the antibacterial/antifungal function is semi-permanently imparted to wood.
2. Prior Art
The utilization of wood is often restricted by its characteristic features of "flammability," "biodeterioration" and "dimensional instability".
A number of proposals have been made for improving these properties. To impart antibacterial/antifungal and preservative functions, a variety of treatments have been proposed and practiced in the art.
One of the currently available preservative (antibacterial/antifungal) treatments is the application of creosote oil which has long been employed. The creosote oil is a mixture of aromatic hydrocarbons as a main component and numerous compounds. The creosote oil-treated wood found application as railroad ties and utility poles. As a wood preservative, the creosote oil has the advantages of low cost, ease of penetration and effective preservative action and the disadvantages of giving off stenchful and eye and skin-irritant fumes. When dissolved out, the creosote oil can contaminate the surrounding soil and leach into rivers or lakes where it is toxic to fish. In many aspects, the creosote oil imposes a heavy burden to the environment.
The exterior wood which is most frequently used at present is CCA-impregnated wood. The CCA is a water-soluble chemical agent containing copper (Cu), chromium (Cr) and arsenic (As) as basic chemicals. It is in wide-spread use over the world because the wood treated therewith exhibits excellent performance. However, the inclusion of such elements as chromium and arsenic draws concerns about the safety of this chemical agent. In some countries, the use of CCA is restricted. Since CCA-treated wood pieces are often used as play instruments in parks and schools, the influence of CCA on human bodies through skin contact is also a concern. With respect to oral toxicity, the toxicity of CCA must be carefully examined because CCA-treated wood pieces are used as outdoor play instruments for children. While acid rain now becomes one of the causes of global environmental damage, it is expected that CCA chemicals can be leached out of the wood with acid rain. When CCA-treated wood is burned for disposal, arsenides are sublimated from the CCA agent as diarsenic trioxide and released into the air, and oxides of copper and chromium are released as microparticulates. From the safety standpoint, the Environmental Protection Agency of USA rules that CCA-treated wood must not be burned in the open air or fireplaces. In UK, it is required that if one makes a fire in the open air using CCA-treated wood, the distance from houses must be at least 100 meters.
Since creosote oil and CCA impose heavy burdens on the environment as mentioned above, careful consideration must be made on their use at the present day when global environmental problems are highlighted. It is DDAC (didecyldimethylammonium chloride) that enjoys a rapidly growing share as a preservative agent for impregnation. This agent contains a metal salt. An increasing demand for DDAC is expected since DDAC avoids the problem associated with the disposal of wood which is of most concern in the case of CCA. Regrettably, the effect of DDAC does not last long and its semi-permanent fixation has not been realized. Besides, metal salts of naphthenic acid are used as a preservative of the surface treatment type and also as a chemical agent for pressure impregnation. They are known to be fully safe, but the retention of their effect is yet a problem. For this reason, the metal naphthenates are mainly used for the treatment of foundations (see "Wood Science Series 5--Environment," Kaisei-sha, 1995). The social demand for antibacterial/antifungal and bactericidal properties is very high as demonstrated in Japan during the prevalence of group food poisoning by pathogenic colibacillus O-157 in 1996. It is urgently required to develop a wood treating agent capable of retaining its effect semi-permanently and remaining safe.
Making investigations to develop high functional wood which is improved in environmental pollution and leach, we found a method for modifying wood by impregnating wood with a silicon alkoxide and subjecting it to hydrolysis and polycondensation to form silicon oxide fixedly within the wood cell walls (inter- and intracellular spaces) whereby various functions such as anti-rotting, dimensional stability and flame retardance are imparted to the wood as reported in the Journal of the Japan Wood Research Society, 38, 11, 1043 (1992). This method relies on the sol-gel process of metal alkoxide in that the starting solution of metal alkoxide-water-alcohol-catalyst converts into a sol of metal oxide through hydrolysis and self-polycondensation of the metal alkoxide. With further progress of reaction, the solution converts into a gel. When this reaction is carried out within wood cells, the inorganic matter based on metal oxide is incorporated into wood.
However, the recent research works revealed that the process of compositing wood with metal oxide largely depends on the rate of hydrolysis of a particular metal alkoxide used and that the distribution of metal oxide in wood cells largely varies with processing conditions. In the event of compositing wood with inorganic matter using a silicon alkoxide having a low rate of hydrolysis and subsequent polycondensation, for example, if the wood used is a moisture-conditioned piece of wood (water in wood is all bound water and present solely in cell walls), the hydrolysis and polycondensation reaction of the silicon alkoxide does proceed solely within the cell walls where bound water is present, resulting in inorganic matter-composited wood in which cell cavities are empty. This composited wood maintains the advantages of wood including light weight, strength and heat insulation, that is, it is modified wood which is provided with rot resistance, dimensional stability and flame retardance while maintaining the porous feature of wood (see the Journal of the Japan Wood Research Society, 39, 3, 301 (1993)). If the wood used is a water-saturated piece (not only cell walls but also cell cavities are full of water), there results in inorganic matter-composited wood in which not only cell walls but also cell cavities are filled with silicon dioxide (see the Journal of the Japan Wood Research Society, 39, 3, 301 (1993)).
If the metal alkoxide used is changed, there is obtained inorganic matter-composited wood having a completely different distribution. If a titanium alkoxide having a high rate of hydrolysis and subsequent polycondensation is used, for example, titanium oxide forms solely in cell cavities in the case of a moisture-conditioned piece. In the case of a water-saturated piece, oxide forms solely on the outer surface of the piece, but no metal oxide is composited in the interior of the piece. See the Journal of the Japan Wood Research Society, 39, 3, 308 (1993).
Based on these findings, we further investigated the relationship between the intracellular distribution of metal oxide and the function imparted thereby, and found that selective compositing of metal oxide in cell walls is a key for a minor amount of metal oxide formed to exert effective functions (see Wood Industry, 50, 9, 400 (1995)). Such compositing is possible with inorganic matter-composited wood based on silicon oxide resulting from silicon alkoxide. once distributed in wood cell walls, silicon oxide is not leached out in water or other solvents and semi-permanently fixed within cell walls.